When light passes through a transparent material, its speed decreases in proportion to the refractive index of the material. The refractive index of certain materials will vary in the presence of electric fields and/or heat. In a typical conventional electro-optic modulator, light passes through a waveguide made of lithium niobate (LiNbO3). By selectively applying an electric field to the waveguide, the light passing through the waveguide will slow, thus changing the phase of the light exiting the waveguide. By varying the electric field according to a digital data signal, the light exiting the waveguide becomes a modulated carrier wave that carries the digital data signal.
In a Mach-Zehnder modulator (sometimes called a “Mach-Zehnder interferometer” or simply “MZM”), an input light signal is split into two waveguides. For example, FIG. 1 discloses a conventional Mach-Zehnder modulator 1 having waveguide upper arm 110 and waveguide lower arm 120. Upper arm 110 has segment 112, made of LiNbO3, that is coupled to high speed data signal 113. High speed data signal 113 creates an electric field affecting segment 112 proportional to high speed data signal 113. Similarly, lower arm 120 has segment 122, made of LiNbO3, that is coupled to high speed data signal 123. High speed data signal 123 creates an electric field affecting segment 122 proportional to high speed data signal 123. In a “push-pull” modulator, high speed data signal 123 is the inverted version of high speed data signal 113.
The light signal of carrier wave 100 splits into light signals 111 and 121, which pass through upper arm 110 and lower arm 120 respectively. As light signal 111 passes through segment 112, its phase changes according to the electric field created by high speed data signal 113. Similarly, as light signal 121 passes through segment 122, its phase changes according to the electric field created by high speed data signal 123. Light signals 114 and 124 are rejoined to produce modulated light signal 130.
Ideally, when there are no electric fields created by high speed data signals 113 and 123, light signals 114 and 124 remain in phase; that is, the sine wave of light signal 114 crosses 0° at the same time the sine wave of light signal 124 crosses 0, and the modulator is said to be operating at its “working point.” Thus, when high speed data signals 113 and 123 are applied to segments 112 and 122 respectively, the resulting modulated light signal 130 will have a recognizable wave form that a receiver can accurately interpret to extract the original data represented by high speed data signals 113 and 123.
However, for a variety of reasons, including manufacturing imperfections, temperature changes, mechanical stresses, and mechanical vibrations, phase shifts in the upper arm 110 and/or lower arm 120 cause light signals 114 and 124 to be out of phase, making it difficult or impossible to demodulate light signal 130 accurately. In order to “tune” light signals 114 and 124 back to the working point, a typical prior art system will add a “dither tone” of known amplitude and frequency, monitor the output signal for the presence of the dither tone, and adjust the bias voltage (that is, the baseline voltage applied to segments 112 and 122) to bring the system back to a known working point. Periodically checking the output of a signal and adjusting the bias voltage accordingly helps keep the output signals 114 and 124 in phase. Alternatively, because the refractive index of certain materials will vary in the presence of heat, the phase of one arm can be tuned by adjusting heater 116 on the top arm.
FIG. 2 discloses a conventional quadrature phase shift keying (QPSK) modulator 2 that essentially consists of two (child) Mach-Zehnder modulators 1a and 1b (with elements of each numbered as in the modulator 1 in FIG. 1) in parallel with one another, nested together to form a third (parent) Mach-Zehnder modulator 1c. In this configuration, Mach-Zehnder modulator 1a performs an in-phase (I) modulation and Mach-Zehnder modulator 1b performs a quadrature phase (Q) modulation; that is, the Q phase is 90° out of phase with respect to the I phase. Thus, the working points for 1a and 1b (the “child arms”) are 0° and the working point for 1c (the “parent arm”) is 90°. This configuration, commonly referred to as an IQ modulator, includes an additional heater 126. Modulator 2 can be dynamically tuned using the same general mechanisms as described for the modulator in FIG. 1.
A LiNbO3 modulator can be phase tuned using dither tones and adjusting bias voltage on both the top and bottom arms of the modulator. Because the phase change has a linear dependence on the voltage/electrical field, phase modulation and control is straightforward and mathematically resolvable. In contrast, silicon photonics (SiP) modulators typically use dithering and heaters on the waveguides for phase tuning. Because the phase change has a quadratic dependent to the heating voltage, phase modulation and control is not straightforward and not easily mathematically resolvable. Further complicating tuning, SiP modulators may use a single arm phase heater to simplify both the chip design and the driver circuit design.
What is needed, therefore, is a mechanism for locking in the phases of a SiP modulator that will work irrespective of the initial difference between the phases.